1. Field of the Invention
This invention relates to a phase shift mask inspection apparatus and, more particularly, to an apparatus for detecting defects of a phase shift mask used for circuit pattern transfer.
2. Description of the Related Art
FIG. 6 shows the structure of a conventional photomask which has a transparent mask base 1 and metal layers 2 formed into desired patterns. When this photomask is uniformly irradiated with light, the light passes through the mask portions which include no metal layer 2 and which consist of mask base 1 portions alone but the light cannot pass through the other mask portions including the metal layers 2 disposed on the mask base 1. The light passing through the photomask therefore has an intensity distribution such as that shown in FIG. 7.
FIG. 8 schematically shows a conventional inspection apparatus for detecting defects in this kind of photomask. A photomask 3 on which a pair of identical patterns are formed is placed on an X-Y stage 4 which has a certain transparency. Inspection light is incident from the backside of the X-Y stage 4, travels through the stage, and irradiates the photomask 3. A pair of photodetectors 5a and 5b are disposed above the photomask 3 in correspondence with the pair of identical patterns and are connected to an inspection processing section 6.
The light which has passed through the respective patterns of the photomask 3 is detected by the photodetectors 5a and 5b, respectively, and their images are processed in the inspection processing section 6, thereby comparing and examining these two patterns. The X-Y stage 4 is moved on an X-Y plane by a stage position control section 7 to effect pattern comparison inspection over the whole surface of the photomask 3.
Since the patterns of the conventional photomask 3 is directly converted into intensities of transmitted light, the optical images received by the photodetectors 5a and 5b are processed so that a defect in one of the patterns of the photomask 3 can be discriminated by using a differential signal from these images. For example, if a metal layer defect 13c occurs in one of the two patterns 13a and 13b on the photomask 3 as shown in FIG. 9, optical images received from the photomask portion along the line A--A of FIG. 9 by the two photodetectors 5a and 5b are as shown in FIG. 10A and FIG. 10B, respectively. That is, in the optical image shown in FIG. 10B, the light intensity distribution is changed due to the metal layer defect 13c. A differential component of these optical images is extracted by the inspection processing section 6 to obtain a differential signal having a higher intensity corresponding to the defect portion as shown in FIG. 10C, thereby discriminating the pattern defect.
The conventional photomask shown in FIG. 6 is thus inspected. Recently, with the increase in transfer pattern density, use of a phase shift mask has been proposed. If the patterns become finer, bending of light into a shadow zone by the diffraction increases and resolution is reduced. A phase shift mask is used to reduce the influence of light bending into the shadow zone.
This kind of phase shift mask has, as shown in FIG. 11, a transparent mask base 21, metal layers 22 provided on the mask base 21 and formed into desired patterns, and phase members 23 formed on the mask base 21 filling appropriate spacings between adjacent metal layers 22. Each phase member 23 is formed of a glass having the same transmittance as the mask base 21 and a thickness such that a phase difference of a half wavelength occurs between the light transmitted through the base 21 and the phase member 23 and the irradiation light transmitted through the base 21 but not transmitted through the phase member 23. Light L1 passing through a surface of the mask where no phase member 23 exists and bending into the adjacent portion covered with the metallic layer 22 and light L2 passing through the phase member 23 and bending into the same portion covered with the metallic layer 22 therefore cancel each other, thereby improving the fine pattern transfer resolution.
In this type of phase shift mask, however, since the phase member 23 has the same transmittance as the mask base 21, a defect in the phase member 23 cannot appear as a change in the optical images obtained by the photodetectors 5a and 5b of the inspection apparatus shown in FIG. 8. That is, a defect in the phase member 23 of the phase shift mask cannot be discriminated by the conventional inspection apparatus shown in FIG. 8.